I think flag4 has the best advice. Shadow and gather information on escorts and then try a night surface attack. This early in the war, you should be able to pull it off. That is one of
the most satisfying accomplishments in SH3, especially when you don't even have to dive to escape and the escorts are throwing starshells out and searching for you in the totally wrong spot.
I think a daylight submerged attack in that shallow of water borders on suicide against an escorted convoy, even early war.
Maybe if there was only one escort and you get in prime position on the seaward side, you might be able to pull off a submerged attack. Shoot and run for deeper water and then become a quiet hole in the water when the escort comes sniffing around looking for some revenge. Type II's are pretty good at that.
I don't think anyone would fault you if you let this convoy go and waited on easier pickings. It's gonna be a long war.